Currently, the circuits included in wireless communication systems must be capable of managing numerous modes of operation. With the escalation in communication standards, communication terminals must be capable of using the most appropriate available network, whatever the physical location. Communications standards differing with regard to aspects such as the modulation of the signal or the width of the channels, the circuits must exhibit great flexibility.
This flexibility is most naturally afforded through the digital processing of the signal performed by virtue of DSP (Digital Signal Processing) in which various algorithms can be implemented in the form of programs and thus chosen freely by the system itself as a function of the conditions of use. By contrast, the analogue part of the circuits is more specific to a given task, and more difficult to adapt to various conditions. This is why systems designers choose to convert the signals into the digital domain as soon as possible. To this end, the signal can be digitized in intermediate frequency (that is to say after a first conversion of the carrier frequency of the signal from the radiofrequency domain—that is to say from 700 MHz to 2.5 GHz—to frequencies of the order of a few tens to a few hundreds of MHz) as described in the article by T. O, Salo, S. J. Lindfors, T. M. Hollman, J. A. L. Järvinen and K. A. I. Halonen, 80-MHz bandpass ΔΣ modulators for multimode digital IF receivers, IEEE. J. of Solid-State Circuits, vol. 38, No. 3, p. 464 (2003) rather than in baseband (after removal of the signal carrier).
A bandpass ΔΣ modulator carries out this analogue-digital conversion of a high-frequency signal with a high resolution, making it a good candidate for direct digitalization into intermediate frequency.
A bandpass ΔΣ modulator carries out a conversion of a high-frequency analogue signal to digital with a high resolution, and solely on a narrow frequency band, thereby rendering it particularly suited to the digitization of a modulated signal. Moreover, the sampling operation can also be used to decrease the frequency of the signal a second time. This makes it possible to decrease the constraints placed on the clock frequency of the modulator, which without this ought to operate at least at three or four times as high a frequency as the intermediate frequency envisaged. After conversion to the digital domain, the signal is converted digitally into baseband and demodulated.
Bandpass ΔΣ modulators are customarily limited by low quality coefficients and non-linearities introduced by the resonators based on LC discrete passive elements or using a transistor coupled to a capacitor (Gm-C). To alleviate these limitations, it has been proposed to use quartz resonators, electromechanical resonators as described in the article by X. Wang, Y. P. Xu, Z. Wang, S. Liw, W. H. Sun, L. S. Tan, A bandpass sigma-delta modulator employing micro-mechanical resonator, Proceedings of the 2003 International Symposium on Circuits and Systems (ISCAS'03), p. I-1041 (2003), surface acoustic wave resonators (R. Yu and Y. P. Xu, Bandpass Sigma-Delta modulator employing SAW resonator as loop filter, IEEE Trans. On Circuits and Systems I: Fundamental theory and applications, vol. 54, p. 723 (2007), or indeed even Lamb wave resonators (M. Desvergne, P. Vincent, Y. Deval and J. B. Bégueret, RF Lamb wave resonators in bandpass delta-sigma converters for digital receiver architectures, IEEE Northeast workshop on Circuits and Systems (NEWCAS 2007), p. 449 (2007)), which are distinguished by their quality coefficients of the order of 1000. The diagram of such a converter is given in FIG. 1, it more precisely entails a reception system using a bandpass analogue-digital converter, and including a resonator M(s) on the circuit.
All these components exhibit an electrical response which as a first approximation can be approximated by the so-called Butterworth-Van Dyke (BVD) diagram as described in the article by K. S. Van Dyke, The piezo-electric resonator and its equivalent network, Proc. IRE, vol. 16, p. 742 (1928) and represented in FIG. 2 illustrating this diagram equivalent to an electromechanical or acoustic resonator.
Intrinsically, components of this type exhibit variations of the phase of their admittance ranging from 90° (capacitive behaviour before resonance and after antiresonance) to −90° (inductive behaviour between resonance and antiresonance), as illustrated in FIG. 3, with two zero-crossings of the phase, the first at resonance and the second at antiresonance. Thus, two feedback loop tuning conditions are possible, thereby leading to an instability of the circuit (M. Desvergne, P. Vincent, Y. Deval and J. B. Bégueret, RF Lamb wave resonators in bandpass delta-sigma converters for digital receiver architectures, IEEE Northeast workshop on Circuits and Systems (NEWCAS 2007), p. 449 (2007)). To avoid this, and to get back to a resonator exhibiting the same transfer function as a resonator of customary LC type, it is necessary to compensate the static capacitance C1 represented in FIG. 2, of the resonator.
The compensation of the static capacitance of the resonator can be done by constructing a capacitor bridge such as illustrated in FIG. 4. This circuit exhibits in practice the equivalent of a negative capacitance Cc placed in parallel with each resonator. In order that the compensation be complete, it may therefore be necessary for the capacitance Cc to be matched with the static capacitance C1 of the resonator. The transfer function illustrated in FIG. 5 is then generated, which now exhibits only a single resonance.
From a practical point of view, the production of sigma-delta modulators is prone to manufacturing dispersions, which are manifested by dispersions in the value of the static capacitance C1 of the resonator. At the present time, the production of such a circuit with a capacitance exactly matched with the static capacitance of the resonator remains problematic. More generally, this type of problem arises in the case where there is a need to resort, in a circuit composed of heterogeneous elements (that is to say each using a different technology), to capacitance values which must be rigorously matched with capacitance values encountered in other modules of the circuit. The solution generally adopted is to resort to a sorting of the components by values and to assemble together only those which have close values, or to use variable elements making it possible to compensate for drifts. All these solutions turn out to be expensive since at the minimum they require electrical tests of parts of the circuit in the course of manufacture or assembly in order to evaluate the necessary correction.
In the case of the filter mentioned previously, in addition to variations in the capacitance values, the manufacturing dispersions also induce variations in the operating frequency. Elements for adjusting the operating frequency are therefore customarily added to the circuit. This can be done by modifying the frequency of the oscillator defining the operating frequency of the circuit.
By pushing this idea further, it has even been proposed to utilize the frequency tunability of a bandpass analogue-digital converter to include the operation of channel selection within the converter. Just where a channel selection filter, characterized by a narrow bandpass but a large rejection, was placed before the analogue-digital converter, it has been proposed to remove it and to use the resonator of the converter as filtering element as described in the article by O. Shoaei and W. M. Snelgrove, Design and implementation of a tunable 40 MHz-70 MHz Gm-C bandpass ΔΣ modulator, IEEE Trans. on Circuits and Systems II: Analog and Digital Signal processing, vol. 44, p. 521 (1997), thereby making it possible to simplify the reception architecture.
Ultimately, a direct conversion of the radiofrequency signal has even been envisaged recently, so as to allow still greater simplification of the architectures, since the mixer used to carry out the operation of decreasing the frequency of the carrier is removed in this case also. In this case, it is the filtering element inserted into the loop of the analogue-digital converter which selects the useful channel. In order for this to be achievable, it is necessary to render the resonator frequency-agile.
A means of rendering resonators frequency-agile consists in using materials whose elastic properties can vary as a function of a DC electric field that it is possible to apply as a supplement to the radiofrequency signal, as illustrated in FIG. 6. The best materials available hitherto are those of the perovskite crystalline family, and notably BaxSr1-xTiO3(BST), which is the best candidate therefor, as described by S. Gevorgian, A. Vorobiev and T. Lewin, DC-field and temperature dependent acoustic resonances in parallel-plate capacitors based on SrTiO3 and Ba0.25Sr0.75TiO3 films: experiment and modeling, J. Appl. Phys. 99, 124112 (2006), making it possible to obtain a relative variation of resonant frequency approximating 6%. This type of material suffers, however, from a major defect: the dielectric properties of this material also depend on the applied static electric field, thereby causing variations of the static capacitance of the resonator which are concomitant with frequency variations, as illustrated in FIG. 7.
This makes it problematic to compensate the static capacitance of the resonator, since it is necessary to be able to ensure at any moment that the capacitances of the compensation circuit of FIG. 4 are matched perfectly with the static capacitances of the resonators. With respect to the first problem area of matching capacitance values despite manufacturing dispersions, this precise case is compounded with an additional difficulty on account of the variability during operation of the elements considered. It is therefore necessary to be able to ensure that the components to be matched exhibit the same initial dispersions, but also the same variations during operation. At the present time, no solution to this problem has been afforded.
In the absence of a solution to this second problem, other types of tunable resonators, not exhibiting the variation of static capacitance as a function of frequency excursion, have been proposed in the literature. It has for example been proposed in the patent application from the authors R. Sinha, L. R. Carley, D. Y. Kim, Devices having a tunable acoustic path length and methods for making the same, patent 2009/0289526 A1, to produce a composite resonator, using a piezoelectric layer (for example of AlN) connected to the radiofrequency circuit (and therefore not exhibiting any impedance variations), adjoined with an agile material, such as BST, linked to a DC voltage generator intended to modify its effective elastic properties. Such a structure is represented in FIG. 8. The agility being obtained only indirectly by a modification of the speed of propagation of the acoustic waves (of the order of 1% for BST) in only part of the resonator, the resonant frequency variations that may be expected therefore remain relatively modest.
A possibility of achieving a larger frequency variation consists in utilizing another physical mechanism: a modification of the conditions at the electrical limits applied to a piezoelectric film (by connecting the film to an exterior variable capacitance for example) makes it possible to envisage relative variations in frequencies proportional to the coefficient of electromechanical coupling of the waves utilized, as described in the article: A. A. Frederick, H. H. Hu and W. W. Clark, Frequency tuning of film bulk acoustic resonators, Proc. of SPIE, vol. 6172, p. 617203 (2006).
For an AlN/AlN composite resonator, this makes it possible at best to envisage relative frequency variations of 3%, that is to say half as much as BST resonators. The use of materials possessing piezoelectric properties far superior to AlN, such as for example LiNbO3 or KNbO3 would in theory make it possible to envisage relative frequency variations of 25%, but to date these materials remain difficult to integrate into composite resonators (P. Muralt, J. Antifakos, M. Cantoni, R. Lanz and F. Martin, Is there a better material for thin film BAW applications than AlN?, 2005 Ultrasonics Symposium Proceeding, p. 315 (2005)).
Whatever the physical mechanism employed in the last two types of variable resonators, even if these structures exhibit a constant value of static capacitance whatever the frequency excursion, nonetheless it is necessary, with a view to the use of these resonators in the filter of a filtering analogue-digital converter, to use compensation capacitors whose value is rigorously equal to the static capacitance of the resonator, thus leading to the first stated problem.
For composite structures using two piezoelectric layers such as represented in FIG. 9, the driving of the frequency is done by way of a variable capacitor whose value must be adjusted relatively with respect to the capacitance of the adjusting layer (the lower piezoelectric layer in FIG. 9), as illustrated by FIG. 10. One of the means of precisely driving components of this type consists in producing a bank of switched capacitors so as to toggle the output of the lower piezoelectric layer to a capacitance equal to xCtpl, where x is a factor that may be greater or less than 1 (for example, x lies between 1/100 and 100) and Ctpl the capacitance created by the frequency tuning layer. In order for this to be possible without the introduction, through technological dispersions or environmental variations, of a shift between the values necessary for the driving of the resonator and those provided by the capacitor bank, it is also necessary to match the capacitances of the bank to those of the resonator tuning layer. This type of component therefore generates a third problem area: being capable of providing capacitances that always remain proportional to a reference capacitance, whatever the manufacture drifts or the drifts in the capacitance values encountered during the operation of the circuit.